Logically, yes it would.
Cosplay Pikachu is a brand new form that is introduced in Pokémon Omega Ruby & Alpha Sapphire. ~ Serebii
If you look at Pikachu's entry in the Hoenn Pokédex, you will see that Cosplay Pikachu is listed as a "form" of Pikachu. This means that Cosplay Pikachu is still a Pikachu and not a different species of Pokémon.
It cannot breed or evolve. ~ Serebii
Interesting. Cosplay Pikachu cannot evolve. But regular Pikachu can evolve. As we have discovered in the previous paragraph, Cosplay Pikachu is still a Pikachu. It then stands to reason that Cosplay Pikachu behaves like a not-fully-evolved Pokémon. Therefore, Eviolite will work on Cosplay Pikachu.